Next month, the European Space Agency's Rosetta space probe will catch up with a comet, which it has been chasing across the solar system for a decade. Once there, the Rosetta probe will take pictures, make measurements and drop a probe down to the comet's surface. Last January, NPR science correspondent Joe Palca reported that for more than two years the spacecraft had shut off most of its instruments, including the radio for communicating with Earth, in order to save energy. Hear now the story Joe did for ALL THINGS CONSIDERED on why the shutdown was necessary.

JOE PALCA, BYLINE: Rosetta went into hibernation in June, 2011. There was an automated timer onboard that had specific instructions to wake the spacecraft up two and half years later and send a radio signal back to Earth. There was no reason to think Rosetta wouldn't wake back up; it was operating just fine when it went to sleep. But still, you can imagine the relief at mission control in Darmstadt, Germany when the signal came in.

(CHEERING)

PALCA: Paolo Ferri was one of the people cheering. He's head of mission operations for the European Space Agency.

PAOLO FERRI: Two a half years were tough enough, but the last 45 minutes were very, very, very tough. I think I don't want to repeat that again.

PALCA: So why did Rosetta have to hibernate? Well, the spacecraft gets its power from solar arrays that convert light from the sun into electrical energy. But in order to catch up with the comet it's chasing, Rosetta had to go out nearly half a billion miles from the sun. And Ferri says that far away, the sun is not very bright.

FERRI: Although we have large arrays, we didn't have enough energy, electrical energy, to keep all systems active.

PALCA: So they left on the automated timer, of course, but they also left on some heaters.

FERRI: Because you can imagine at those distances from the sun, we had to keep the unit from freezing. And so the little energy that remained on the solar arrays, which was on the order of a few hundred watts, was used to operate these thermostat heaters to keep the spacecraft as warm as possible.

PALCA: Rosetta's main scientific mission starts this August, when it catches up with the comet it's been chasing since 2004, a comet with a name that doesn't exactly roll off the tongue - or at least not my tongue.

FERRI: Well, we pronounce it Churyumov-Gerasimenko.

PALCA: Churyumov-Gerasimenko.

FERRI: Right.

PALCA: Anyway, if everything goes according to plan, once Rosetta catches Churyumov-Gerasimenko, it will fly alongside for two years as the comet swings around the sun. It will also release a probe that is intended to land gently on the comet's surface. But before any of that can happen, Ferri says in May, Rosetta will have to make some crucial maneuvers.

FERRI: At the moment, we are still flying very, very fast compared to the comet. We have to brake.

PALCA: So that means Rosetta will have to fire its rocket engine. And it needs to change course slightly. Right now, Rosetta's not pointed in exactly the right direction, and that means a second rocket firing.

FERRI: If something goes wrong with those and we can't, for whatever reason, deform them, then we don't have a mission.

PALCA: Not that there's any reason to think the rocket engine won't work properly. It has in the past. But until it does, Ferri and his Rosetta colleagues will have to suffer through a few more tense moments. Joe Palca, NPR News.

INSKEEP: And we can report the time for tension is nearly over. Claudia Alexander is the U.S. Rosetta project scientist.

CLAUDIA ALEXANDER: Everything is on target for an August 6 rendezvous. But we have an intriguing surprise.

INSKEEP: Alexander says that instead of a lumpy, round shape, as you would imagine a comet to be, the comet looks to some people like a rubber ducky. That's a quote. Transcript provided by NPR, Copyright NPR.